Return of naturally sourced Pb to Atlantic surface waters

Abstract

Anthropogenic emissions completely overwhelmed natural marine lead (Pb) sources during the past century, predominantly due to leaded petrol usage. Here, based on Pb isotope measurements, we reassess the importance of natural and anthropogenic Pb sources to the tropical North Atlantic following the nearly complete global cessation of leaded petrol use. Significant proportions of up to 30-50% of natural Pb, derived from mineral dust, are observed in Atlantic surface waters, reflecting the success of the global effort to reduce anthropogenic Pb emissions. The observation of mineral dust derived Pb in surface waters is governed by the elevated atmospheric mineral dust concentration of the North African dust plume and the dominance of dry deposition for the atmospheric aerosol flux to surface waters. Given these specific regional conditions, emissions from anthropogenic activities will remain the dominant global marine Pb source, even in the absence of leaded petrol combustion.

Figure 1. Collection locations of samples used in this study.

Panel (a), sampling locations for unfiltered surface seawater (circles) and aerosols (brown lines) along the middle part of the GEOTRACES section GA02 (2nd leg; cruise PE321) and along the GEOTRACES section GA06 (cruise D361). Brown circles indicate surface seawaters affected by mixing with Amazon River outflow in the western tropical Atlantic (WTA). Green arrows show the main meteorological regimes, while black lines denote the position of the ITCZ (Inter Tropical Convergence Zone) during each cruise. The map was produced using Ocean Data View. Panels (b) and (c) show aerosol optical depth (550 nm) averaged over the duration of the cruises PE321 (section GA02, 2nd leg) and cruise D361 (section GA06) respectively. The position of the North African mineral dust plume over the Atlantic Ocean can be clearly identified by higher values (darker colours). Visualizations were produced using the Giovanni online data system, NASA GES DISC with data from the MODIS instrument on board the Terra satellite.

Figure 2. Lead concentrations and isotopic compositions results for surface seawater samples.

Panel (a) displays Pb concentration results, while panels (b) and (c) display ²⁰⁶Pb/²⁰⁷Pb and ²⁰⁸Pb/²⁰⁷Pb ratios respectively, as a function of latitude. Circles denote the results for unfiltered seawater samples, and squares are results for filtered seawater (western Tropical Atlantic only). Green and purple shaded fields denote the position of the North African dust plume during the 64PE321 and D361 cruise, in the western and eastern Tropical Atlantic (WTA and ETA) respectively. Uncertainty assessed through replicate analyses of in-house seawater standards quoted at 1 s.d. for Pb concentrations and 2 s.d. for Pb isotope ratios.

Figure 3. A plot of ²⁰⁶Pb/²⁰⁷Pb versus ²⁰⁸Pb/²⁰⁷Pb for surface water and aerosol samples for source assessment.

Panel (a) ²⁰⁶Pb/²⁰⁷Pb versus ²⁰⁸Pb/²⁰⁷Pb for unfiltered surface seawater from GEOTRACES sections GA02 (2nd leg) and GA06 and aerosols (total digests and leachates; GA06). Shown for comparison are the Pb isotope compositions of (1) the major potential Pb sources to the tropical Atlantic (coloured fields2324252627282930) with recent North American aerosols denoted by light blue diamonds, (2) surface seawaters collected in the region during the 1980s and 1990s (grey squares89), as well as (3) a mixing line between anthropogenic Pb (Pb_anth) transported by easterly winds and mineral dust derived Pb (Pb_min). Inset panel (b), displays an enlargement of ²⁰⁶Pb/²⁰⁷Pb versus ²⁰⁸Pb/²⁰⁷Pb ratios for unfiltered surface seawater only. Quoted uncertainties (2 s.d.) for the seawater and aerosol data are assessed as through replicate analyses of in-house seawater standards (seawater) or by propagation of uncertainty associated with blank corrections and measurement protocols (aerosols).

Figure 4. Comparison of the Pb contents and isotope composition of Amazon influenced to the remainder of WTA surface waters.

Salinity versus ²⁰⁸Pb/²⁰⁷Pb ratios (panel a), and Pb concentrations (panel b) for unfiltered surface seawater samples collected in the western tropical Atlantic (GEOTRACES section GA02, 2nd leg). The samples affected by mixing with Amazon River water are distinguished as brown circles, while other samples are marked in green. Quoted uncertainties in ²⁰⁸Pb/²⁰⁷Pb ratios (2 s.d.) and Pb concentrations (1 s.d.) are assessed through replicate analyses of in-house seawater standards.

Figure 5. Results of mineral dust derived Pb proportion estimates for the aerosol total digests.

(a) comparison between the proportions of mineral dust derived Pb (Pb_min) in the total digests of aerosol samples, as estimated using the isotope mass balance versus the enrichment factor (EF) based approaches. The grey line denotes a 1:1 relationship. Panel (b) the proportion of Pb_min in the total aerosol digests estimated using the isotope mass balance approach plotted versus atmospheric Al concentrations, as a proxy for atmospheric mineral dust abundance. The black contours denote constant anthropogenic Pb (Pb_anth) concentrations for the atmosphere (in ng m⁻³), calculated for mineral dust with Pb/Al ratio of 2.09 × 10⁻⁴ (ref. 44). Uncertainty in the Pb_min proportion estimates by the isotope mass balance based approach is assessed by propagation of the full range of ²⁰⁶Pb/²⁰⁷Pb ratios compiled for North African mineral dust through the calculations.

Figure 6. The relationship between solubility and proportion of mineral dust derived Pb.

Results of the fractional solubility of Pb in the leaching procedure relative to the total digests as a function of estimated proportion of mineral dust derived Pb in each aerosol sample. Displayed Pb_min proportion estimates are derived using the isotope mass balance approach, with uncertainty assessed by propagation of the full range of ²⁰⁶Pb/²⁰⁷Pb ratios compiled for North African mineral dust through the calculations. Uncertainty of the fractional solubility estimates (1 s.d.) is assessed by propagation of the variability of the sampling blank through the calculation.